The ever-increasing consumption of nonrenewable energy sources and global concerns toward environmental protection has compelled scientific communities to search for sustainable energy storage technologies.
Supercapacitors are one of the most promising energy storage devices, as a result of their intrinsic performance advantages. Supercapacitors store energy in terms of charges by either ion adsorption (electrochemical double layer capacitors (EDLCs)) or surface faradic reactions (pseudocapacitors) on electrodes.
High-performance supercapacitors should be designed to satisfy a set of required properties, such as high specific capacitance, long cyclic stability and large rate capability. The high-power density of supercapacitors over other energy storage systems, such as lithium ion batteries, makes them suitable for a variety of applications where high bursts of power are instantly required, as in electric vehicles and wind turbines. Moreover, their fast charging-discharging and long-term cyclic stabilities make them attractive for power supplies in portable electronic devices. As the advances of mobile/portable devices are progressing to incorporate additional functionalities, as in flexible/wearable energy technologies, it is compelling to develop supercapacitors which deliver high power under mechanical deformations for long time cycles. Performance of supercapacitors can be improved by optimizing the intrinsic properties of electrode materials as well as rationally engineering their electrode designs. From the material properties' perspective, high electronic/ionic conductivities are essential for reducing capacitance losses, particularly at high scan rates/current densities. Accordingly, materials that are electrically conductive (for electron transport), yet structurally nanoporous (for ion transport), are highly desired.
From the electrode designs' perspective, electrodes should be integrated on metal current collectors with mechanically/chemically stable interfaces (ideally, binder-free), which will ensure a long cycle life as well as high electronic/ionic conductivities. Moreover, enlarging the surface area of electrodes will lead to improved specific capacitance owing to the increased amount of charges stored on their surfaces.
Recently, substantive efforts have been focused on developing nanostructured electrode materials to meet the aforementioned requirements by bridging their superior material properties to more efficient electrode designs. For example, an array of electrochemically active one-dimensional (1D) nanowires directly integrated on current collectors can offer multiple advantages. Their large surface areas with small diameters provide facile access and short diffusion paths for electrolyte ions, which can improve capacitances. Moreover, the direct integration of nanowires (active electrode materials) on metallic substrates (current collectors) without the use of binding materials can enhance mechanical robustness while reducing capacitive losses at their interfaces.
In addition to the 1D nanostructures, two-dimensional (2D) transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have recently being considered as promising capacitive materials due to their structural advantages. For example, the intrinsically layered structure of 2D TMDs enables the facile incorporation of ions in between 2D layers separated by sub-nanometer physical gaps, which favors a fast ionic adsorption/transport through them, which are free to expand. Their large surface area is another contributing factor for enhancing capacitance via the EDLC mechanism.
Despite the projected advantages, most 2D TMDs do not present sufficiently high electrical conductivities, unlike zero-bandgap graphene, which hampers the direct applications of stand-alone 2D TMDs for supercapacitors. Unlike the single materials-based approaches, incorporating multiple 1D and 2D materials with well-defined dimensions and distinct functionalities are anticipated to offer synergic advantages. These efforts include the combination of 2D TMDs with highly conductive carbonaceous 1D or 2D materials as well as conductive polymers, constructing hybrid composites for electrodes. Some demonstrations include 2D WS2 incorporated with 1D carbon nanotubes (CNTs) and reduced graphene oxide (rGO), as well as 2D MoS2 composites with carbons and conductive polymers. Despite enhanced specific capacitance in some cases, these 2D TMDs based composite materials still suffer from limited performances. For example, significant capacitance decays are observed after a few hundred-to-thousand charge-discharge cycles, which are attributed to the poor structural integrity at the interfaces of randomly integrated dissimilar nanomaterials.
Accordingly, 2D TMDs have emerged as promising capacitive materials for supercapacitor devices owing to their intrinsically layered structure and large surface areas. In addition, hierarchically integrating 2D TMDs with other functional nanomaterials has recently been pursued to improve electrochemical performances. However, the supercapacitors known in the art often suffer from limited cyclic stabilities and capacitance losses due to the poor structural integrity at the interfaces of randomly assembled materials.
Accordingly, what is needed in the art is an improved supercapacitor having increased cyclic stability and reduced capacitances loses.